Although most animals are more solid than fluid, what happens when you put many of them together can be strikingly fluidic. Above you see the black aquatic worm, Lumbriculus variegatus, which must keep moist to stay alive. An individual worm will die within an hour of being removed from the water, but, in a group, the worms can survive far longer. They do so, in part, by acting like a viscoelastic fluid, a material with both solid (elastic) and fluid (viscous) properties.
In small groups, the worms squirm tightly together to minimize their collective surface area and prevent themselves from drying out. But in larger groups, the worm blobs begin sending out feelers, searching for more advantageous circumstances. In the top image, you can see this causes three of the blobs to ultimately merge into an even bigger one. The worm collective can also “liquify”, allowing the blob to change shape and tackle obstacles like flowing through a pipe. (Image and research credit: Y. Ozkan-Aydin et al.; via Science)
This is the second post in our series on collective motion. Check out the first post here.